I was messing with the tuning on my 72 Chevy C20 with dana 60 and I did a power brake on gravel and the first time it spun the passenger side tire and a slight turn on the drivers side. Today it turned the drivers side. The tag on the rear end says 4.10 and I counted the revs per 2 turns of the tire and got a little over 4 on the drive shaft.

Does the fact that it will turn both mean that I have a limited slip differential? If so I assume it has clutches. Is there a wrong fluid you can use in this rear end? When I got the truck I changed all the fluids and used regular rear end lube. Could this slip and cause a problem with towing?

Just because it spins both tires sometimes does not mean that it has a LSD. Many times a standard differential will also spin both tires. The best way to tell exactly what you have is to remove the cover and look inside. This will require a oil change, but there is no other positive way to know what you have. You could have a LSD with worn out clutches, or you could have a standard differential with a galded cross shaft. I have seen a bent axle tube cause both tires to spin.

I think it might not be limited slip. When I raise the rear and spin one tire the other spins backward. Isn't that an open diff?

99% of the time that is correct. Could be the slim possibility of a LSD with totally fried clutches, but not likely.

Any open diff will spin both tires if the loading is equal on both tires(say you are positioned just right on a crowned paved road), or if the surface gives easily like a gravel road, or on wet pavement.

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